Heterodyne interferometers are used to measure the phase shift of a beam of light caused by an optical phase shifter. An optical bypass line whose length is to be measured may function as an optical phase shifter. Heterodyne interferometers are generally conventional.
In a heterodyne interferometer, the beam of a light source, usually a laser, is passed through a beam splitter to two acousto-optical modulators. The two acousto-optical modulators are triggered by frequencies f1 and f2 which are typically in the MHz range. The beams of light at the output of the acousto-optical modulators are shifted here by a corresponding frequency with respect to the original light frequency.
The two frequency-shifted beams of light are then sent back to a beam splitter via mirrors and combined, one of the two beams being delayed via an optical phase shifter. This may be accomplished via materials which shift the phase of light or with which the speed of the light with respect to air is reduced. According to another embodiment, the light is deflected by additional mirrors and must therefore travel through an optical bypass. After the two beams of light have been combined again by the beam splitter, e.g., in the form of a semitransparent mirror, the light is sent to a receiver containing a photodetector and usually an amplifier.
The two beams of light cause interference and generate a beat frequency known as a heterodyne frequency fHet in the receiver. This frequency is calculated as follows:fHet=|f1−f2|.The phase of this signal, based on the phase angle of an electric signal of frequency fHet obtained by mixing f1 with f2, corresponds to the phase shift of the optical phase shifter that is to be measured.
The analog signal available at the output of the receiver is sent to a downstream A/D converter which generates a digital signal. The conversion is then performed at a sample rate of frequency fa. The digital signal is then sent to an analyzer unit for further processing.
In the case of the heterodyne interferometers described above, generating frequencies f1, f2 and fa during operation may result in great fluctuations in heterodyne frequency fHet=|f1−f2| because the oscillators may have frequency drift with temperature and also with aging. Another disadvantage is that blanking frequency fa does not form an integral ratio with heterodyne frequency fHet and is not even constant.